1. Field of the Invention
The present invention relates to a three-dimensional shape measuring system and a three-dimensional shape measuring method for measuring a three-dimensional shape of an object to be measured (hereinafter, called as “measurement object”).
2. Description of the Related Art
Heretofore, there has been known a non-contact three-dimensional shape measuring apparatus for measuring a three-dimensional shape of the entirety of a measurement object by measuring a three-dimensional shape of a part of the measurement object in a non-contact state multiple times at a position around the measurement object, and by using the measurement results. The conventional non-contact three-dimensional shape measurement technique is disclosed in e.g. Japanese Unexamined Patent Publication Nos. Hei 9-145319 (D1), 2000-2520 (D2), and 2000-105111 (D3).
In the technologies disclosed in D1 through D3, slit light is projected toward a measurement object, and the light reflected on the measurement object is received by an area sensor. Then, the three-dimensional shape of the measurement object is derived by calculating a distance between a certain point on the surface of the measurement object, and a targeted pixel of the area sensor where the reflected light from the certain point on the surface of the measurement object is incident, based on the position of the targeted pixel on the area sensor.
D1 is designed to measure the shape of the measurement object with a higher resolution than the resolution defined by the pixel pitch. Specifically, in the case where the surface of the measurement object is scanned with slit light having a bandwidth corresponding to the widths of “n” pixels, the slit light is shifted by the pitch corresponding to one pixel at each sampling cycle. Effective light receiving data is obtained from one pixel by performing the sampling operation “n” times, and interpolation computation is performed by using the “n” light receiving data. Each of the pixels of the area sensor has a field of view, i.e. the center position of the field view in a strict sense, on the surface of the measurement object. The interpolation computation is performed to obtain a timing (a time centroid or a point of time when the light receiving amount of a targeted pixel is maximum) at which the optical axis of the slit light passes a certain measurement point, specifically, a center position for measurement. The position of a measurement point on the surface of the measurement object is calculated, based on a relation between the projecting direction of the slit light at the calculated timing, and the incident direction of the slit light onto the targeted pixel.
In D2, an operation of cyclically scanning a measurement object to capture an image of the measurement object is performed multiple times while changing the projecting direction of slit light. Also, the intensity of the slit light is changed in each of the scanning operations. Then, a scanning operation in which the obtained sampling data neither reaches a saturated level nor a black level is specified among the scanning operations. Then, a centroid in the specified scanning operation i.e. a centroid on a time axis of a distribution concerning multiple light receiving data obtained by multiple sampling operations executed by a one-time scanning operation is derived with respect to each of the pixels. Then, the shape of the measurement object is derived by using the centroids.
In D3, a non-destructive readable image sensor is used as an image sensor for capturing a reflected light component of slit light projected onto a measurement object. Specifically, multiple light receiving data obtained with different exposure times are acquired at respective sampling cycles from each of the pixels of the image sensor by a non-destructive reading method i.e. a method of retaining an electric charge in the image sensor, as far as a reset signal is not supplied. Unsaturated light receiving data is selected from the multiple light receiving data obtained with the different exposure times with respect to each of the pixels. Then, calculated is a timing at which the optical axis of the slit light passes the field of view on the surface of the measurement object corresponding to each of the pixels, using the light receiving data. Then, the position of the measurement point on the surface of the measurement object is calculated, based on a relation between the projecting direction of the slit light at the calculated timing, and the incident direction of the silt light onto the corresponding pixel.
The conventional three-dimensional shape measuring apparatuses need improvement in securing precision in measuring the three-dimensional shape of the measurement object. For instance, there is proposed an arrangement of calculating the position of the respective measurement points on the surface of a measurement object, using light receiving data obtained with a different exposure time i.e. a different light receiving period at each sampling cycle with respect to each of the pixels, in combination with the arrangement disclosed in D1.
The above arrangement requires a procedure of calculating a time centroid in correlation to a center timing in each of the light receiving periods, based on an assumption that the light receiving data is obtained in each of the sampling operations with the same light receiving period. In the above arrangement, the time centroid calculated with respect to each of the pixels may be displaced from the timing at which the optical axis actually passes each of the pixels. Accordingly, the three-dimensional shape of the measurement object measured by the aforementioned measuring technique may include a measurement error resulting from the displacement.